std::ranges:: find_first_of

From cppreference.com

Defined in header `<algorithm>`
Call signature
template < std:: input_iterator I1, std:: sentinel_for < I1 > S1, std:: forward_iterator I2, std:: sentinel_for < I2 > S2, class Pred = ranges:: equal_to , class Proj1 = std:: identity , class Proj2 = std:: identity > requires std:: indirectly_comparable < I1, I2, Pred, Proj1, Proj2 > constexpr I1 find_first_of ( I1 first1, S1 last1, I2 first2, S2 last2, Pred pred = { } , Proj1 proj1 = { } , Proj2 proj2 = { } ) ;	(1)	(since C++20)
template < ranges:: input_range R1, ranges:: forward_range R2, class Pred = ranges:: equal_to , class Proj1 = std:: identity , class Proj2 = std:: identity > requires std:: indirectly_comparable < ranges:: iterator_t < R1 > , ranges:: iterator_t < R2 > , Pred, Proj1, Proj2 > constexpr ranges:: borrowed_iterator_t < R1 > find_first_of ( R1 && r1, R2 && r2, Pred pred = { } , Proj1 proj1 = { } , Proj2 proj2 = { } ) ;	(2)	(since C++20)

1) Searches the range

first1

last1

for any of the elements in the range

first2

last2

, after projecting the ranges with proj1 and proj2 respectively. The projected elements are compared using the binary predicate pred .

2) Same as (1) , but uses r1 as the first source range and r2 as the second source range, as if using ranges:: begin ( r1 ) as first1 , ranges:: end ( r1 ) as last1 , ranges:: begin ( r2 ) as first2 , and ranges:: end ( r2 ) as last2 .

The function-like entities described on this page are algorithm function objects (informally known as niebloids ), that is:

Explicit template argument lists cannot be specified when calling any of them.
None of them are visible to argument-dependent lookup .
When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

Parameters

first1, last1	-	the range of elements to examine (aka haystack )
first2, last2	-	the range of elements to search for (aka needles )
r1	-	the range of elements to examine (aka haystack )
r2	-	the range of elements to search for (aka needles )
pred	-	binary predicate to compare the elements
proj1	-	projection to apply to the elements in the first range
proj2	-	projection to apply to the elements in the second range

Return value

Iterator to the first element in the range [ first1 , last1 ) that is equal to an element from the range [ first2 , last2 ) after projection. If no such element is found, an iterator comparing equal to last1 is returned.

Complexity

At most S * N applications of the predicate and each projection, where
(1) S = ranges:: distance ( first2, last2 ) and N = ranges:: distance ( first1, last1 ) ;
(2) S = ranges:: distance ( r2 ) and N = ranges:: distance ( r1 ) .

Possible implementation

struct find_first_of_fn
{
    template<std::input_iterator I1, std::sentinel_for<I1> S1,
             std::forward_iterator I2, std::sentinel_for<I2> S2,
             class Pred = ranges::equal_to,
             class Proj1 = std::identity,
             class Proj2 = std::identity>
    requires std::indirectly_comparable<I1, I2, Pred, Proj1, Proj2>
    constexpr I1 operator()(I1 first1, S1 last1, I2 first2, S2 last2, Pred pred = {},
                            Proj1 proj1 = {}, Proj2 proj2 = {}) const
    {
        for (; first1 != last1; ++first1)
            for (auto i = first2; i != last2; ++i)
                if (std::invoke(pred, std::invoke(proj1, *first1), std::invoke(proj2, *i)))
                    return first1;
        return first1;
    }
 
    template<ranges::input_range R1, ranges::forward_range R2,
             class Pred = ranges::equal_to,
             class Proj1 = std::identity,
             class Proj2 = std::identity>
    requires std::indirectly_comparable<ranges::iterator_t<R1>,
                                        ranges::iterator_t<R2>,
                                        Pred, Proj1, Proj2>
    constexpr ranges::borrowed_iterator_t<R1>
        operator()(R1&& r1, R2&& r2, Pred pred = {},
                   Proj1 proj1 = {}, Proj2 proj2 = {}) const
    {
        return (*this)(ranges::begin(r1), ranges::end(r1),
                       ranges::begin(r2), ranges::end(r2),
                       std::move(pred), std::move(proj1), std::move(proj2));
    }
};
 
inline constexpr find_first_of_fn find_first_of {};

Example

Run this code

#include <algorithm>
#include <iostream>
#include <iterator>
 
int main()
{
    namespace rng = std::ranges;
 
    constexpr static auto haystack = {1, 2, 3, 4};
    constexpr static auto needles  = {0, 3, 4, 3};
 
    constexpr auto found1 = rng::find_first_of(haystack.begin(), haystack.end(),
                                               needles.begin(), needles.end());
    static_assert(std::distance(haystack.begin(), found1) == 2);
 
    constexpr auto found2 = rng::find_first_of(haystack, needles);
    static_assert(std::distance(haystack.begin(), found2) == 2);
 
    constexpr static auto negatives = {-6, -3, -4, -3};
    constexpr auto not_found = rng::find_first_of(haystack, negatives);
    static_assert(not_found == haystack.end());
 
    constexpr auto found3 = rng::find_first_of(haystack, negatives,
        [](int x, int y) { return x == -y; }); // uses a binary comparator
    static_assert(std::distance(haystack.begin(), found3) == 2);
 
    struct P { int x, y; };
    constexpr static auto p1 = {P{1, -1}, P{2, -2}, P{3, -3}, P{4, -4}};
    constexpr static auto p2 = {P{5, -5}, P{6, -3}, P{7, -5}, P{8, -3}};
 
    // Compare only P::y data members by projecting them:
    const auto found4 = rng::find_first_of(p1, p2, {}, &P::y, &P::y);
    std::cout << "First equivalent element {" << found4->x << ", " << found4->y
              << "} was found at position " << std::distance(p1.begin(), found4)
              << ".\n";
}

Output:

First equivalent element {3, -3} was found at position 2.

Compiler support
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Standard library
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Feature test macros (C++20)
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Concepts library (C++20)
Metaprogramming library (C++11)
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Technical specifications
Symbols index
External libraries

find_first_of	searches for any one of a set of elements (function template)
ranges::adjacent_find (C++20)	finds the first two adjacent items that are equal (or satisfy a given predicate) (algorithm function object)
ranges::find ranges::find_if ranges::find_if_not (C++20) (C++20) (C++20)	finds the first element satisfying specific criteria (algorithm function object)
ranges::find_end (C++20)	finds the last sequence of elements in a certain range (algorithm function object)
ranges::search (C++20)	searches for the first occurrence of a range of elements (algorithm function object)
ranges::search_n (C++20)	searches for the first occurrence of a number consecutive copies of an element in a range (algorithm function object)